1. Field of the Invention
The present invention relates to a vibration type motor which utilizes resonance of a vibration member.
2. Related Background Art
In recent years, vibration wave (vibration type) motors called ultrasonic wave motors or piezoelectric motors have been developed and put into practical applications by the present applicant and others. As is well known, a vibration wave motor is a non-electromagnetic driving type motor that picks up, as continuous mechanical motion, high-frequency vibrations produced by applying AC voltages to electro-mechanical energy conversion elements such as piezoelectric elements, electrostrictive elements, or the like. Since the principle of operation of this motor H as already been explained in many patents, such as Japanese Patent Laid-Open Application No. 3-289375, and the like, a detailed description thereof will be omitted.
FIG. 10 is a side view of a conventional rod-like ultrasonic wave motor and a diagram showing the wiring layout for supplying voltages to and picking up an output voltage from piezoelectric elements which are mounted in the motor. A vibration member 101 is included in the rod-like ultrasonic wave motor, and comprises a coupled body of piezoelectric elements or electrostrictive elements and an elastic member. A rotor 102 contacts the upper portion of the vibration member 101.
A piezoelectric element portion of the vibration member 101 comprises A-phase driving piezoelectric elements a1 and a2, B-phase driving piezoelectric elements b1 and b2, and a vibration detection piezoelectric element s1. When an A-phase application voltage is applied to a metal plate sandwiched between the A-phase piezoelectric elements a1 and a2, and a B-phase application voltage is applied to a metal plate sandwiched between the A-phase piezoelectric elements b1 and b2, the piezoelectric elements are driven. The outer surfaces of the A-phase piezoelectric elements a1 and a2, and B-phase piezoelectric elements b1 and b2 are connected to the ground potential (GND). Similarly, one surface (the B-phase side in FIG. 10) of the vibration detection piezoelectric element s1 also is connected to the GND potential, and a signal is picked up from the surface opposite to the GND side. The signal pickup surface side of the vibration detection piezoelectric element s1 contacts a metal block, which is insulated from the GND potential by an insulating sheet. Hence, an output voltage corresponding to vibrations generated in the vibration detection piezoelectric element s1 is directly obtained from the vibration detection piezoelectric element s1. The resonance frequency or the like is calculated, e.g., based on the magnitude of the output voltage, the phase difference from the driving voltage, and the like.
FIG. 11 shows a driving circuit for the ultrasonic wave motor.
A- and B-phase signals are applied via driving electrodes A-d and B-d (see FIG. 1) for applying AC voltages to the piezoelectric elements or electrostrictive elements. The circuit shown in FIG. 11 includes a control circuit (to be referred to as a control microcomputer hereinafter) 11 for controlling driving of the motor, an oscillator (e.g., a voltage-controlled oscillator (VCO) or the like) 2 for generating an AC voltage, a 90.degree. phase shifter 3, switching circuits 4 and 5 for switching power supply voltages using the AC voltages from the oscillator and phase shifter, inductance elements 6 and 7 for attaining impedance matching with the motor, and a phase difference detector 8 for detecting a signal phase difference .theta..sub.A-S between a driving signal A and a vibration detection signal S.
Conventionally, the power supply voltage to be switched by the switching circuits 4 and 5 may be as high as several 10 volts, but an ultrasonic wave motor that can be driven at low voltages also is available since the piezoelectric elements have been improved.
In an ultrasonic wave motor that can be driven at low voltages, the power supply may comprise a dry cell, battery, or the like, and the power supply voltage of which may change with time. Consequently, the input power to the motor changes, and the motor cannot be stably driven.
As described above, when the motor is driven using the switching circuits shown in the prior art, the input power to the motor changes in response to drifts of the power supply voltage, and it is impossible to drive the motor by constant input power.